Evaluation of different raw earthen plasters stabilized with lime for bio-building exploitation

The building sector generates around 5-8% of greenhouse gas emissions (GHG)1 and the disposal of C&D waste at the end-of-life is a high environmental cost. The raw earth is a sustainable construction material with low embedded energy, available locally. It is the most ancient technique of construction, studied in recent years to reduce the environmental life cycle impact of buildings. Clay is responsible for the earth plastic behaviour and represents the binder that keeps together silt and sand grains. Earth sets through drying without chemical reactions, so it could be reinserted into the nature. At the same time, earthen constructions do not withstand weathering and develop lower mechanical performances compared with those which exploit hydraulic binders. We investigated the possibility of improving these characteristics by stabilizing earthen products with the addition of small amounts of lime preserving clay as eco-friendly binder and the full end-of-life recyclability. Four earths with different binder properties – two kaolin clays, one illite clay and a smectite – were characterized and analysed at the lab scale. The change of the Atterberg limits on adding lime was determined. The clay modification and potential pozzolanic reactions with lime were measured by X-ray diffraction. Two earths were selected to be tested as plasters, adding sand and lime in order to reduce shrinkage and increase water resistance. Cracking and adhesion tests were run for all mixtures. The results show decreasing performances for all the plasters mixtures stabilized with lime, especially in the presence of an expandable clay fraction. The water resistance is improved for the stabilized mixtures that require less sand to reduce cracking and swelling. Lime is better used as a surface finish and plaster, not as stabilizer, because reacting with clay it increases the plastic limit and raises the water demand to obtain a plastic and workable mixture.